Former sweetheart, model, dancing round apartment, briefly (3,5)
I believe the answer is:
old flame
'former sweetheart' is the definition.
(I've seen this before)
'model dancing round apartment briefly' is the wordplay.
'dancing' indicates anagramming the letters.
'round' means one lot of letters goes inside another.
'apartment' becomes 'flat' ('flat' can be a synonym of 'apartment').
'briefly' means to remove the last letter.
'model' is an anagram of 'oldme'.
'flat' with its last letter taken away is 'fla'.
'oldme' placed around 'fla' is 'OLD FLAME'.
(Other definitions for old flame that I've seen before include "Former lover" , "still 1 across [ALIGHT], it seems" , "Ex-lover - the fire still burns" , "Former partner" , "Ex" .)